1. Field
Various embodiments of the invention relate to a photographing lens used in an electronic still camera or a digital video camera, and more particularly, to a bright wide-angle lens having a wide viewing angle by using an inner focus method.
2. Related Art
Recently, optical image-forming devices, such as digital cameras or digital camcorders, using a solid-state image pickup device, such as a charge coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS) image sensor, have been rapidly developed and have become widespread. Accordingly, high performance photographing lens having a small and lightweight structure is required.
In the field of optical systems using the solid-state image pickup device, the optical systems need to achieve not only high resolution but also high magnification, all the while being small and lightweight. Also, because the technical expertise of consumers of cameras is continuously increasing, a lens design method that focuses on certain functions of a wide-angle lens, a telephoto lens, and a zoom lens, while satisfying miniaturization, high performance, and auto-focusing is continuously being researched.
A wide-angle lens generally uses a two-group structure called a retro-focus lens. In the two-group structure, a first group has negative refractive power and a second group has positive refractive power, and a back focal length is similar to or longer than a focal length. Here, a total length of the two-group structure is increased as a distance between the first and second groups is increased. But the back focal length is required to be equal to or larger than a predetermined distance, even as a latter group portion is moved or as the latter group portion and a former group portion are simultaneously or separately moved to perform focusing. Thus, the decrease in the total length of the two-group structure is limited. Moreover, in a bright lens, a plurality of lenses are moved during focusing for aberration correction, and thus a focusing response is not great. Also, a lens group performing the focusing is typically heavy thereby increases the load on a driving unit, and thus the focusing may not be quickly performed.